Electrochemical compressor systems may be, for example electrolysers, which by applying a potential, in addition to producing, for example water and oxygen from water, compress these gases at the same time under high pressure.
In addition, electrochemical compressor systems, such as for example electrochemical hydrogen compressors, are also known, to which gaseous molecular hydrogen is supplied and the latter is compressed electrochemically by applying a potential. This electrochemical compression is available particularly for small quantities of hydrogen to be compressed, since mechanical compression of hydrogen would be considerably more expensive here.
Electrochemical systems are known, in which an electrochemical cell stack is constructed with layering of several electrochemical cells, which are separated from one another in each case by bipolar plates. The bipolar plates thus have several tasks:                electrical contacting of the electrodes of the individual electrochemical cells and conveying the current to the adjacent cell (series connection of the cells),        supplying the cells with reactants, such as for example water or gases and, for example removal of the reaction gas produced via an appropriate distributor structure,        conveying the heat being produced during generation in the electrochemical cell, and        sealing off of the various media ducts or cooling ducts with respect to one another and externally.        
The bipolar plates have openings for cooling or media supply and media discharge for media supply and media discharge from the bipolar plates to the actual electrochemical cells (these are for example MEA (Membrane Electron Assembly) having a gas diffusion layer, for example made from a metal mat, orientated in each case towards the bipolar plates).
Difficulties regularly result here particularly with regard to the gas diffusion layer. It has been conventional hitherto to design the seal between the bipolar plates or between bipolar plates and the electrochemical cell in that an elastomer seal is placed, for example in a groove of the bipolar plate. By exerting compressive strain (for example by means of tension bands) on the electrochemical cell stack, pressing of the seal then takes place, as a result of which a sealing effect should be achieved for the openings.
It is now a problem for the inserted gas diffusion layer that it may be designed as a fibre mat (with metal fibres) or metal mesh. Fibre mats which are conventional in industry have a theoretical thickness of, for example 1 mm, but the manufacturing tolerance is ±100 μm. The metal fibres which construct the mat are themselves only slightly resilient. In addition, it is also not recommended to compensate production tolerances of the fibre mat by compressing the mat, since the gas permeability of the mat layer is thus severely diminished and hence operation of the electrochemical cell is restricted. On the other hand however, it is necessary to exert a minimum pressure on the entire gas diffusion layer by the bipolar plate, so that there is an adequate passage of current through the gas diffusion layer. It can thus be summarised that for the current elastomer seals, either a non-perfect seal or non-optimum operation of the electrochemical cell was thus to be accepted. In addition, particularly for electrochemical cells operated using molecular hydrogen, permeation losses of H2 occur which diffuse through the elastomer seal.
As a first aspect, the object of the present invention is therefore to achieve a secure seal of the openings in an electrochemical cell stack with as low costs as possible.
This is achieved by an electrochemical compressor system according to the invention.
By providing bead arrangements, which are resilient at least in some regions, for sealing the openings, a secure seal is achieved over a long resilient path of the bead arrangement. Openings are thus understood to mean virtually any region to be sealed in the present application. This may be, for example a passage opening for a reaction fluid (for example H2 or water) or a cooling agent. However, it may also be, for example the electrochemically active region, in which for example the gas diffusion layer is arranged or screw holes are provided. The resilient bead arrangement always permits compensation of production tolerances of, for example gas diffusion layers, in a wide tolerance range and nevertheless provision of an optimum sealing effect.
A very advantageous embodiment of the invention envisages that the bead arrangement is designed for microsealing with a thin coating having a thickness between 1 μm to 400 μm. The coating is advantageously made from an elastomer, such as silicone, viton or EPDM (ethylene/propylene-diene terpolymer), application is preferably effected by a screen-printing process, tampon-printing process, spraying or by CIPG (cured in place gasket; that is elastomer introduced at the site of the seal as liquid which is cured there.). These measures ensure that, for example hydrogen diffusion is reduced to an extremely low degree by the seal, since the height of the permeable material is adapted to a minimum. Attempts are thus made not to recover additional geometric height, but only to provide roughness compensation for microsealing.
A further advantageous embodiment of the invention envisages that the bead arrangement contains a full bead or a half bead. It is thus also possible within a bead arrangement to provide both forms, since depending on the course of the bead arrangement in the plane, other elasticities may prove useful, for example that in narrow radii a different beading geometry is useful than for straight courses of the bead arrangement.
An advantageous development of the bead arrangement envisages that the bead arrangement is designed at least in some regions as a half bead constructed around the electrochemically active region and open is around the latter in some regions. It is thus attached so that it is open towards the high-pressure side, thus ensuring that by increasing the internal pressure, the increase in contact pressure of the bead against the sealing surface of the next bipolar plate (or the membrane lying therebetween) is achieved. Since the electrochemical compressor stack is stabilised externally by end plates which are held together using tension bands or the like, yielding of the stacked individual plates is only possible to a limited extent. There is no “elastic expansion” of the entire arrangement but only a rise in contact force in regions of the seal so that there is even self-stabilisation of the seals or of the entire arrangement. The half bead is thus designed so that by increasing pressure in the system (this internal pressure may be over 200 bar, preferably over 700 bar, particularly preferably over 1,000 bar up to 5,000 bar) in the electrochemically active region, surface pressure directed in the direction of the electrochemical compressor stack is increased so that tightness problems are excluded and thus a quasi “self-stabilising” system is provided with regard to the seal.
A further advantageous embodiment envisages that the bead arrangement is made from steel. Steel has the advantage that it can be processed very cost-effectively using conventional tools, in addition, for example methods for coating steel with thin elastomer layers are well tested. The good elasticity properties of steel facilitate the good design of the long resilient sealing region of the invention according to the invention. There is thus the particular possibility that the bead arrangement is attached to the bipolar plate. There is thus firstly the possibility that the bipolar plate is designed as a whole as a steel moulding (which is possibly provided with a coating for corrosion resistance or conductivity in some regions). However, it is also possible that the bipolar plate is designed as a composite element of two steel plates with a plastic plate lying therebetween. However, in each case the good manufacturing possibilities of steel may be utilised, it is possible to make the bead arrangement within a manufacturing step which is taking place in any case (for example embossing of a flow field, that is a “stream field”). Very low costs are thus produced, also no additional sources of error are provided by extra components, such as for example additionally inserted elastomer seals.
Nevertheless, it is also possible according to the invention to provide the bead arrangement made from other metals, such as for example steel, nickel, titanium or aluminium and alloys thereof. The choice, which metal is to be preferred, thus depends, for example also on the required electrical properties or the required degree of corrosion resistance.
It thus becomes possible to adapt the compression characteristic of the bead, for example to a gas diffusion layer. However, this does not have to apply only to gas diffusion layers, the bead line may generally be well adapted to components having low elasticity. The beaded seal can be designed flexibly and hence in addition can be applied well and without high retrofitting costs for all producers of electrochemical compressor systems.
A further advantageous embodiment envisages that the bead arrangement has a stopper, which limits compression of the gas diffusion layer to a minimum thickness. It is thus an incompressible part of the bead arrangement or a part, the elasticity of which is very much lower than that of the actual bead. This ensures that the degree of deformation is limited in the bead region, so that there cannot be complete flat pressing of the bead.
A further advantageous embodiment envisages that the bead arrangement is arranged on a component which is separate from the bipolar plate. This is particularly favourable when the bipolar plates consist of material which is unsuitable for bead arrangements. The separate component is then placed on the bipolar plate or integrated by adhesion, clicking-in, welding-in, soldering-in or moulding-in, so that overall a sealing connection is produced between the separate component and the bipolar plate.
Finally, a further advantageous embodiment envisages that the bead arrangement is designed from an elastomer roll. Such a bead can be applied by a screen-printing process or tampon printing. It serves both for microsealing and for macrosealing. The roll also assumes the function of path adaptation on a gas diffusion layer.
A further advantageous development envisages that the electrochemical compressor system is designed as an electrolyser. Here, water introduced on one side of the electrochemical cell is cleaved electrochemically into molecular hydrogen and oxygen. Membranes made from Nafion or similar proton-conducting systems are used for this, but separators may also be used, which contain, for example PTFE foams soaked with potassium hydroxide. Also porous ceramic structures, soaked with potassium hydroxide, are possible separators, for example structures based on Nextel or also hydroxide-conducting structures. The contact forces (surface pressures of the seal in the main direction of the electrochemical cell stack) may lie between 0.1 and 200 N/mm2, preferably over 10 N/mm2, particularly preferably over 50 N/mm2.
A further advantageous development envisages that the electrochemical compressor system is a hydrogen compressor, which oxidises molecular hydrogen introduced on the first side of a proton-conducting electrochemical membrane to H+ and reduces it again on the second side back to molecular hydrogen, wherein the molecular hydrogen there is subjected to a higher pressure on the second side than on the first side due to the sealing and spatial arrangement. The operating temperature should lie here between 0 and 100° C., conceivably also 0-200° C. or 0-550° C. Hydroxide-conducting structures or even known proton-conducting polymer membranes (for example made from Nafion) may be used here as membranes.
Of course other gases may also be compressed correspondingly for a suitable choice of ion conductor, for example oxygen with hydroxide-conducting structures.
Overall it should be remembered that the present electrochemical compressor system should at the very best tolerate very high pressures which are significantly higher than for other electrochemical mechanisms. The prevailing gas pressure in the electrochemically active region should be able to be without leakage losses at least 100 bar, preferably over 200 bar, particularly preferably over 500 bar.
A second aspect of the present invention is concerned with the object of achieving secure sealing of the openings in an electrochemical cell stack with as low as possible costs, wherein also safe supply of media for cooling and for operation of the electrochemical cell (in particular O2 or air or H2) from the openings to cooling cavities or towards the electrochemically active regions of the electrochemical cell should be guaranteed safely. This aspect can be applied to any electrochemical systems, such as for example fuel cells or the above-mentioned electrochemical compressor systems.
This object is achieved in that resilient bead arrangements are provided around the openings of at least one bipolar plate, wherein perforations for conducting liquid or gaseous media are arranged on at least one flank of the bead arrangements. An electrochemical compressor system (or for a fuel cell system) consisting of an electrochemical cell stack (or a fuel cell stack) having layering of several electrochemical cells (or fuel cells), which are separated from one another in each case by bipolar plates, wherein the bipolar plates have openings for cooling or media supply and media discharge for the electrochemical cells (fuel cells) and the electrochemical cell stack (or fuel cell stack) can be placed under mechanical compression strain in the direction of the layering, wherein resilient bead arrangements are provided around the opening of the bipolar plate, wherein perforations for conducting liquid or gaseous media are arranged on at least one flank of the bead arrangements, is thus shown here.
It is thus particularly advantageous that first of all sealing of the openings is generally achieved by the bead arrangement when applying a mechanical pressure in the direction of layering of the electrochemical cell stack, which sealing is cost-effective and provides good tolerance compensation. Specific supply or discharge of cooling agents into corresponding cooling agent cavities and also secured media supply and media discharge is also additionally facilitated by the perforations in the flanks of the bead arrangements. It is no longer necessary that the bead has to be completely interrupted in order to supply or discharge cooling agents or operating media quasi orthogonally to the direction of layering of the electrochemical cell stack (which coincides here with the direction of an interface duct). Hence it is already possible in the production of these bipolar plates to provide the corresponding perforations which lead later to media supply in the finished electrochemical compressor system. It is thus advantageous that such perforations can be easily produced on a large scale, flow resistances and the stiffness of the bead arrangement etc. may be preset precisely by varying the perforations.
In particular cost-effective production of a bipolar plate or of parts of the bipolar plate is possible in that a metal plate is provided with holes first of all and then mechanical shaping of the perforated plate takes place to produce the bead arrangement so that the previously introduced holes are perforations in at least one flank of the bead arrangement. Of course it is however also possible to first emboss the profile of the bipolar plate and then to introduce the perforations, for example using laser processing, punch supply etc.
Hence, it may be said by way of summary that the value of the invention lies in that simplified media supply to the active region of the bipolar plate is possible. “Tunnelling” of a seal is not necessary, since the media supply in this case takes place through the sealing system itself. This is firstly space-saving and secondly facilitates higher volume and weight capacities of the electrochemical cell. The invention is available particularly for metallic bipolar plates for PEM electrochemical cells, which are constructed in most cases from two embossed metal sheets which are flatly connected to one another. The media water, in some cases cooling water, and the gases thus have to be effectively sealed with respect to one another. If the seal of a metallic bipolar plate is designed as a bead construction, the bead is in most cases severely flattened at the points through which media should flow into the active region. Support for the membrane is not present at these points, which may lead to gas leakages (“cross-over”) or to the collapse of the membrane into the supply channel. However, if perforations are introduced into the flanks of the bead, and permit the media, for example hydrogen, air, distilled water, to flow transversely through the bead into the flow field region of the bipolar plate, the bead is able to rest against the membrane uninterrupted. Clean sealing of the media flows is thus achieved. The perforations may thus be designed more advantageously as circles or also as ovals in order not to noticeably change the spring characteristic of the bead. Sealing between the fluid flows occurring in the electrochemical cell is guaranteed by a design of the second metallic plate adapted to the bead construction in the region of media passage. The beads may thus be designed as full beads or half beads. Furthermore, media passage may take place through the bead with connected ducts. This is advantageous especially for guiding the cooling medium. It may thus be guided more easily between the anode and the cathode plates.
A further advantageous development envisages that the perforations in the flank plane may have a circular, oval or angular cross-section. The flow properties of fluids guided through these perforations may be influenced first of all by this shaping and the appropriate number of perforations per flank plane. In addition, the stiffness of the bead arrangement can also thus be controlled for stress in the direction of layering of the electrochemical cell stack, since the corresponding geometrical moments of inertia are also co-influenced by shaping of the perforations.
A particularly advantageous development envisages that a duct is connected to a perforation, wherein the duct is connected to the bead interior and is closed at least towards the bead outer surface. This ensures that the perforations are not guided directly from the bead interior to the outside, but that specific delivery through a duct, for example in the hydrogen gap of the bipolar plate, is possible; the introduction of oxygen into the cathode of the electrochemical cell is thus prevented. It is particularly advantageous in terms of production technology that these ducts may also be co-embossed at the same time as embossing of the bipolar plate (when it consists, for example of metal), alternatively, of course the later or earlier attachment of individual ducts is possible.
A further development envisages that the perforations are open towards the electrochemically active region of the electrochemical cell. This is applied in particular to introduce media, such as hydrogen. Of course different variants next to one another at the same time are also possible in a single bipolar plate, that is those perforations which are connected to ducts and those perforations which have no ducts.
An industrially particularly promising embodiment envisages that the bipolar plate is constructed from two (metal) plates, which has a cavity lying therebetween for cooling agent and/or passing of media gases, such as H2. The interior of this bipolar plate may thus also be divided into segments, for example into those which serve on the one hand for guiding cooling agent and on the other hand for distributing media gases. This segmenting may thus be provided by connecting regions of the two plates, which are designed for example as a welding or soldering.
A further advantageous development envisages that the bead arrangement contains a “full bead” or a “half bead”. For the full bead there is thus the option of providing perforations on one or on both flanks. Whether a half bead or a full bead is required, depends, inter alia, on the required stiffness or also on the geometry of the opening.
The bead arrangement is available particularly for bipolar plates which consist of metals, such as steel, nickel, titanium or aluminium and alloys thereof. The bead arrangements may thus be part of a topography embossed in the bipolar plate. However, it is also possible to arrange the bead arrangement on a component which is separated first of all from the bipolar plate, and is then placed later in particular on bipolar plates made from metal, plastic, graphite or the like or connected to the bipolar plate by adhesion, clicking-in, welding-in, soldering-in or moulding-in.
A further advantageous development envisages that the bead arrangement is coated for microsealing. This guarantees, for example with an elastomer layer which is applied for example by a screen-printing process the outer side of the bead arrangement, that microsealing is provided against media passage. This elastomer coating also has the additional effect that in the case of a polymer membrane placed on this coating, a “floating” or “gliding” fixing is provided, which ensures that this membrane of the electrochemical cell remains fixed on the one hand even for size changes in the region of 10% and on the other hand shows no cracks due to too rigid fixing.
A further advantageous development envisages that an electrochemically active region of the electrochemical cell is arranged in an essentially closed chamber, which is limited essentially annularly laterally by a bead arrangement. This means that a bead arrangement is possible not only for sealing openings of the bipolar plate, but that also “total sealing” of the interior of the electrochemical cell stack is possible.
A particularly advantageous development envisages that the bead arrangements have essentially the same stiffness for stresses in the direction of layering of the electrochemical cell stack in the perforated and the non-perforated flank regions. Adjustment of the same stiffness may thus take place in different ways. It may take place, for example by means of a flank angle which varies along the course of the bead arrangement (for example a steeper flank angle in the perforated flank regions) or by a suitable material distribution (that is for example thicker wall thicknesses in the immediate surrounding region of the perforations). For example steels having a maximum tensile strength of Rm of 300 to 1,100 N per mm2, preferably 600 to 900 N per mm2 may be used. These steels have a modulus of elasticity between 150,000 and 210,000 N per mm2.